The 65-year-old Biden is a veteran of more than three decades in the Senate, and one of his party's leading experts on foreign policy, an area in which polls indicate Obama needs help in his race against Republican rival John McCain.

According to a Democratic official, Biden slowly emerged as Obama's choice across a long day and night of political suspense as other contenders gradually fell away.

The official who spoke did so on condition of anonymity, preferring not to pre-empt a text-message announcement the Obama campaign promised for Saturday morning.

Obama's campaign had invited supporters to sign up for text messages and e-mails notifying them when the announcement was made, and senior campaign officials told FOX the messages will be timed to be received when the nation is awake.

Obama's campaign has managed to keep a tight veil of secrecy over the decision as media and delegates start traveling to Denver for a four-day extravaganza that will end with an acceptance speech by the first black presidential nominee of a major party.

Earlier, Virginia Governor Tim Kaine and Indiana Senator Evan Bayh had been informed that they were out of the running for Obama's vice presidential pick, and had been informed of the same, making Biden the leading contender.